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Date: Fri, 21 Sep 2012 22:46:36 +0900
From: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Avi Kivity <avi@redhat.com>, Rik van Riel <riel@redhat.com>,
        Srikar <srikar@linux.vnet.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        KVM <kvm@vger.kernel.org>, Jiannan Ouyang <ouyang@cs.pitt.edu>,
        chegu vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC 2/2] kvm: Be courteous to other VMs in overcommitted
 scenario in PLE handler
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On Fri, 21 Sep 2012 17:30:20 +0530
Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> wrote:

> From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> 
> When PLE handler fails to find a better candidate to yield_to, it
> goes back and does spin again. This is acceptable when we do not
> have overcommit.
> But in overcommitted scenarios (especially when we have large
> number of small guests), it is better to yield.
> 
> Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
> ---
>  virt/kvm/kvm_main.c |    4 ++++
>  1 files changed, 4 insertions(+), 0 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 8323685..713b677 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1660,6 +1660,10 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  			}
>  		}
>  	}
> +	/* In overcommitted cases, yield instead of spinning */
> +	if (!yielded && rq_nr_running() > 1)
> +		schedule();

How about doing cond_resched() instead?

I'm not sure whether checking more sched stuff in KVM code is a
good thing.

	Takuya